For the first audition for the role, the casting director may want to talk to you before reading you. So, I would suggest that you sit if you want to. It shows confidence and that you are comfortable being in the room. If you are feeling that they just want you to get right into the reading, then no need to sit. If it is a callback, the audition generally moves much faster. You just need to be ready to do your read. Use the chair if it is needed for the scene. If it is not needed, the no need to sit.